In a traditional printer device, a latent image is applied onto a toner image carrier with the assistance of an electrographic process, for example by exposing a photoconductor or by magnetization of a magnetically sensitive layer. Toner agglomerates at the latent image according to the image-related distribution of the electrical charges or of the magnetic poles. The toner is then transferred onto a carrier material, generally a paper web, at the transfer printing location. The toner image is later fixed on the carrier material.
There is a demand in modern printing technology that a single device print the carrier material on both sides with high speed. This operating mode is generally referred to as duplex printing. Further, there is a demand for the operating mode of spot color printing or two-color printing wherein printing is carried out in two colors on at least one side. Moreover, there is a need for a full-color printing with the four process colors. In order, for example, to realize the operating mode of duplex printing, it is known to initially print the continuous carrier web on one side, to then turn the carrier web over and to print the second side with the same printing unit. A web turn-over means is required given this solution. The conveying path of the carrier web through the printer is relatively long and requires a complicated conveyor mechanism for the carrier material that, thus, is also susceptible to malfunction.
A further disadvantage of the known printer is comprised therein that the toner image can smear when it has not yet been fixed on the one side of the carrier web and, thus, the printing quality is diminished or, respectively, a reject is produced. In order to avoid this, an intermediate fixing of the toner image can ensue, as a result whereof, however, the technical outlay becomes great. Moreover, the carrier web must traverse the fixing process a second time after being printed on the second side, as a result whereof is thermally highly stressed, shrinkage problems occur and the registration of the print is diminished. Due to the long conveying path between the first transfer printing location for the first transfer printing of a toner image and the second transfer printing location, adherence to a high registration of the carrier web can only be assured with great technological outlay.
The aforementioned problems also occur given what is referred to as a "twin system" wherein two identical printers are connected in series with one another in order to realize a two-sided printing, i.e. the duplex printing, while turning the carrier web over. In addition to the high costs for two printers, it is also disadvantageous that a large basement surface must be made available and the apparatus control of the two printers must be adapted to one another with relatively high technological outlay.
EP 0 629 931 A1 (Applicant: XEIKON) discloses an electrostatic printer wherein a carrier web is conducted through between a plurality of toner image carrier in vertical direction. Each toner image carrier has a means that generates a toner image. The toner is transferred onto the carrier web at a respective transfer printing location of the toner image carriers. As a result of the both-sided arrangement of toner image carriers along the vertically proceeding carrier web, duplex printing with different toner colors is possible. The known apparatus has a complicated structure, a complicated carrier material guidance and is bulky.
EP 0 433 444 B1 (Applicant: Eastman Kodak Company) discloses a printer wherein a plurality of developer stations are arranged along a photoconductive band as toner image carrier. Each developer station can provide the charge image generated by an illumination station with toner having a predetermined color. The toner image generated on the photoconductive band is then transferred onto the carrier material at a single transfer printing location. This printer is also technologically complicated and can realize only low printing speeds in the various operating modes.
EP-A-0 629 924 discloses a printer device according to the preamble of claim 1. A photoconductor band can also be employed as photoconductor or, respectively, toner carrier.
EP-A-0 742 496 discloses an electrographic printer device, particularly a printer or copier, having at least two essentially identical printing units. A carrier web is conducted through between the printing units arranged lying opposite one another and is printed. The printing units enable a multi-color printing.
Patent Abstracts of Japan, Volume 15, No. 20 (P1154), Jan. 17, 1991, Pub. No. JP-A-02264276, discloses a printer device wherein a repeating mode is realized. This printer device prints single sheets on only one side. The single sheets are moved back and forth in the repeating mode. The photoconductor band is brought into contact both with a developing unit as well as with the single sheets in the one motion direction. When moving the single sheets in the other direction, the photoconductor band is moved away both from the signle sheets as well as from the developing unit.
Therefore, there is a need for an electrographic printer device that works with high printing speed and has a compact structure. Further, high flexibility in view of various types of printing mode should be achieved.